The invention relates to a thermal cutoff switch and in particular to a thermal cutoff switch that is sealed for use in electrochemical batteries to open the conductive path within the battery if the battery overheats.
There are many applications in the prior art where it is necessary to disconnect electrical circuits when temperatures exceed a predetermined amount. For example only, fire alarms need to be triggered upon a certain temperature being reached at a particular location. Further, in the art of electrochemical cells, it is well known that during charging or discharging of the cell, the amount of internal resistance and the rate of current flow all determine the degree of heat generation within the cell.
If the rate of heat dissipation from an electrochemical cell to the environment is smaller than the rate of heat generation inside the cell, the cell temperature will increase steadily. If the current flow is continuously maintained, a point may be reached where the cell explodes. At this point, some of the cell components such as, for example, lithium and free sulfur resulting from reduction of the components, react instantly with each other and all of the energy of the system is released at once. This is an extremely dangerous condition that may occur if the cell temperature is permitted to exceed the melting point of the elements such as sulfur and lithium.
It becomes important therefor to use an inexpensive, simple thermal cutoff switch in such situations that will open the circuit and interrupt the current flow. In some cases, such as with primary battery cells, it may be important to have a high resistance across the open thermal cutoff switch to bleed down battery power when the thermal cutoff switch opens. Such a battery, when discarded, is thereby rendered more benign.
It is also important to provide an inexpensive simply constructed thermal cutoff switch that may be used to open electrical circuits, or close electrical circuits, upon the attainment of predetermined temperatures. In the battery art, it is important to have the thermal cutoff switch of such a size and construction that it can be located within the battery to sense the internal battery temperature. The switch must also be resistant to the electrolytes that are in the battery, and provide a resistance that is inserted in the circuit when the thermal cutoff switch is opened to bleed down the battery power if the battery becomes short circuited.